A split feature on Livesplit that allows you to keep track of times for all missions listed in the split.

To use it, first download Livesplit. Once you do that and open it, right-click and look for Open Splits, then click on Story% Split, which should show after you download the link in this Resource. If you want to close the split, just look further down from Open Splits, and you'll see Close Splits, returning Livesplit to a simple timer.