diff --git a/neovim/config/plugins/diffview.lua b/neovim/config/plugins/diffview.lua
index f9630bb..0f7befd 100644
--- a/neovim/config/plugins/diffview.lua
+++ b/neovim/config/plugins/diffview.lua
@@ -10,3 +10,25 @@ map('<leader>gd', '<Cmd>DiffviewOpen<CR>', 'Diff against index')
 map('<leader>gc', '<Cmd>DiffviewClose<CR>', 'Close diff view')
 map('<leader>gh', '<Cmd>DiffviewFileHistory %<CR>', 'File history')
 map('<leader>gH', '<Cmd>DiffviewFileHistory<CR>', 'Branch history')
+
+-- The base branch name varies per repo (main, master, develop, ...), so ask
+-- the first remote's HEAD what it is rather than hardcoding one.
+local function default_branch()
+  local remote = vim.fn.systemlist('git remote')[1]
+  if not remote or remote == '' then
+    return ''
+  end
+  local ref = vim.fn.systemlist('git symbolic-ref refs/remotes/' .. remote .. '/HEAD')[1]
+  if not ref or ref == '' then
+    return ''
+  end
+  return ref:gsub('^refs/remotes/' .. remote .. '/', '')
+end
+
+map('<leader>gm', function()
+  vim.ui.input({ prompt = 'Diff against: ', default = default_branch() }, function(rev)
+    if rev and rev ~= '' then
+      vim.cmd('DiffviewOpen ' .. rev .. '...HEAD')
+    end
+  end)
+end, 'Diff against branch')