Barney approaches a personal milestone - sleeping with the first woman he hits on seven nights in a row. Elsewhere, Lily and Marshall realise they've been using the same toothbrush.
The gang tries to help Ted when the perfect woman becomes available; Marshall finds a letter written by his younger self.
A new lead in a cold case requires Lisbon to postpone her plans to relocate to Washington. This gives Jane time to wake up to his feelings for her and decide on a course of action.
An FBI agent once romantically involved with Stillman asks the team for help finding a serial killer who has evaded her for 30 years.
In this conclusion of the two-part episode, the Philadelphia PD and the FBI continue their joint operation to catch the serial killer before he strikes again.