diff --git a/src/filepersist.rs b/src/filepersist.rs
index 6334cac..2dc00da 100644
--- a/src/filepersist.rs
+++ b/src/filepersist.rs
@@ -2,6 +2,8 @@ use eyre::Context;
 use git2::Repository;
 use serde::{Deserialize, Serialize};
 use std::path::PathBuf;
+use std::rc::Rc;
+use crate::api::PullRequest;
 
 #[derive(Clone, Debug, Serialize, Deserialize, Default)]
 pub struct BranchPersistenceData {
@@ -10,6 +12,35 @@ pub struct BranchPersistenceData {
     pub stack_tag: Option<String>,
 }
 
+impl BranchPersistenceData {
+    pub fn get_latest_base_commit(&self, pr_opt: Option<&Rc<PullRequest>>) -> Option<(String, u64)> {
+        if let Some(pr) = pr_opt {
+            if let Some(ref base_branch) = self.base_branch {
+                if pr.base() != base_branch {
+                    eprintln!("PR for branch {:?} does not have same base as persistent data!", pr.head());
+                    return None;
+                }
+            }
+
+            // Choose which one to use in the case of disagreement.
+            match (&self.base_commit, pr.base_commit()) {
+                (Some((here_base, here_ts)), Some((remote_base, remote_ts))) => {
+                    if *here_ts >= remote_ts {
+                        Some((here_base.to_owned(), *here_ts))
+                    } else {
+                        Some((remote_base.to_owned(), remote_ts))
+                    }
+                }
+                (Some(a), None) => Some(a.to_owned()),
+                (None, Some((a, b))) => Some((a.to_owned(), b)),
+                (None, None) => None
+            }
+        } else {
+            self.base_commit.clone()
+        }
+    }
+}
+
 pub fn find_persistence_path(repo: &Repository) -> eyre::Result<PathBuf> {
     let persist_path = repo.path().join("git-stack");
     if !persist_path.exists() {
diff --git a/src/graph.rs b/src/graph.rs
index ebc7887..3c2b12b 100644
--- a/src/graph.rs
+++ b/src/graph.rs
@@ -8,6 +8,10 @@ use crate::api::PullRequest;
 
 pub type FlatDep = Vec<(Rc<PullRequest>, Option<Rc<PullRequest>>)>;
 
+pub fn find_pr_for_branch_in_flatdep<'a>(stack: &'a FlatDep, branch: &str) -> Option<&'a Rc<PullRequest>> {
+    stack.iter().find(|(pr, _base_pr)| pr.head() == branch).map(|(pr, _)| pr)
+}
+
 pub fn build(prs: &[Rc<PullRequest>]) -> Graph<Rc<PullRequest>, usize> {
     let mut tree = Graph::<Rc<PullRequest>, usize>::new();
     let heads = prs.iter().map(|pr| pr.head());
diff --git a/src/lib.rs b/src/lib.rs
index c401caa..245e35e 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -6,6 +6,8 @@ pub mod markdown;
 pub mod persist;
 pub mod util;
 
+pub mod rebase;
+
 pub struct Credentials {
     // Personal access token
     token: String,
diff --git a/src/main.rs b/src/main.rs
index 46cfce3..f0992e6 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -13,7 +13,7 @@ use git_stack::filepersist::{
 };
 use git_stack::graph::FlatDep;
 use git_stack::util::loop_until_confirm;
-use git_stack::Credentials;
+use git_stack::{Credentials, rebase};
 use git_stack::{api, git, graph, markdown, persist};
 
 fn clap<'a, 'b>() -> App<'a, 'b> {
@@ -89,13 +89,10 @@ fn clap<'a, 'b>() -> App<'a, 'b> {
         .arg(ci.clone())
         .arg(identifier.clone());
 
-    let rebase = SubCommand::with_name("rebase")
+    let rebase = SubCommand::with_name("rebase-this")
         .about(
-            "Print a bash script to STDOUT that can rebase/update the stack (with a little help)",
-        )
-        .setting(AppSettings::ArgRequiredElseHelp)
-        .arg(exclude.clone())
-        .arg(identifier.clone());
+            "Rebase this branch by replaying it on the latest version of its base.",
+        );
 
     let branch = SubCommand::with_name("branch")
         .about(
@@ -121,7 +118,7 @@ fn clap<'a, 'b>() -> App<'a, 'b> {
         .subcommand(annotate)
         .subcommand(log)
         .subcommand(rebase)
-        .subcommand(autorebase)
+        //.subcommand(autorebase)
         .subcommand(branch);
 
     app
@@ -275,12 +272,23 @@ async fn main() -> eyre::Result<()> {
             }
         }
 
-        ("rebase", Some(m)) => {
-            let identifier = m.value_of("identifier").unwrap();
-            let stack = build_pr_stack(identifier, &credentials, get_excluded(m)).await?;
+        ("rebase-this", Some(_)) => {
+            let head = repo.head()?;
+            if !head.is_branch() {
+                bail!("Current HEAD isn't a branch.");
+            }
+            let current_branch = head
+                .shorthand()
+                .context("No name for the current reference")?;
 
-            let script = git::generate_rebase_script(stack);
-            println!("{}", script);
+            let head_stack_info = load_persistence_data_for_branch(&repo, current_branch)
+                .context("Failed to load branch persistence data for current branch")?;
+
+
+            let identifier = head_stack_info.stack_tag.as_ref().context("No stack tag registered for this branch!")?;
+            let stack = build_pr_stack(&identifier, &credentials, vec![]).await?;
+
+            rebase::rebase(&repo, &stack, head_stack_info, current_branch)?;
         }
 
         ("autorebase", Some(m)) => {
diff --git a/src/rebase.rs b/src/rebase.rs
new file mode 100644
index 0000000..3b98dab
--- /dev/null
+++ b/src/rebase.rs
@@ -0,0 +1,48 @@
+use eyre::{bail, Context, ContextCompat};
+use git2::{BranchType, Oid, Repository};
+use crate::filepersist::BranchPersistenceData;
+use crate::graph::{find_pr_for_branch_in_flatdep, FlatDep};
+
+pub fn rebase(repo: &Repository, stack: &FlatDep, head_stack_info: BranchPersistenceData, current_branch: &str) -> eyre::Result<()> {
+    let pr_opt = find_pr_for_branch_in_flatdep(stack, current_branch);
+
+    // First find out what the old commit was that we based this branch on in the first place!
+    let (base_commit, base_commit_ts) = head_stack_info.get_latest_base_commit(pr_opt)
+        .context("Couldn't get base commit from anywhere")?;
+    eprintln!("old base commit is {base_commit:?} at {base_commit_ts:?}!");
+
+
+    // Now find out what we should use as our new base!
+    // Use whatever we find on the active PR first because that gets changed if an underlying
+    // branch moves...
+    let base_branch_name =  pr_opt.map(|pr| pr.base()).or(head_stack_info.base_branch.as_ref().map(|x| x.as_str()))
+        .with_context(|| format!("Can't find a new base branch for the branch {current_branch:?}"))?;
+
+    let base_branch = repo.find_branch(base_branch_name, BranchType::Local)
+        .context("Couldn't find base branch in git repo")?;
+
+    let current_branch = repo.find_branch(current_branch, BranchType::Local)
+        .context("Couldn't find current branch in git repo")?;
+    let current_branch_commit = current_branch.get().peel_to_commit().context("can't peel current branch to commit")?;
+
+    let new_base_commit = base_branch.get().peel_to_commit().context("")?.id();
+    eprintln!("new base commit likely {new_base_commit}");
+    let old_base_commit = Oid::from_str(&base_commit).context("can't look up old commit")?;
+
+    let merge_base_head_old = repo.merge_base(current_branch_commit.id(), old_base_commit).context("can't find merge base between current branch tip and old base")?;
+    eprintln!("merge base between old and current tip is {merge_base_head_old}.");
+    if merge_base_head_old != old_base_commit {
+        bail!("branch tip drifted away from old base commit?!");
+    }
+
+    let merge_base_head_new = repo.merge_base(new_base_commit, old_base_commit).context("can't find merge base")?;
+    eprintln!("merge base between new and current tip is {merge_base_head_new}.");
+    if merge_base_head_new == new_base_commit {
+        bail!("branch tip is already descended from new base commit?!");
+    }
+
+    // TODO
+    println!("git rebase --onto {new_base_commit} {old_base_commit}");
+
+    Ok(())
+}