In the Season Three premiere, Jake’s girlfriend Jen (Jenna Fischer) would return the engagement ring to him. Knowing Laura wanted Jake back, Jen couldn’t stomach being a home wLaura-Fischerrecker, having grown up in a household with a similar dynamic between her own parents.
With Jen out of the way, Jake is open to reconciling with Laura. But since honesty has been the biggest stumbling block in their marriage, he insists that Laura first hear the full details about his affair that led to their divorce two years ago, (We’d establish that Laura never knew the identity of Jake’s mistress.) Laura comes to discover that Jake’s tryst had been with a prosecution witness in a homicide investigation which Jake and Laura’s two precincts worked on together back in 2014.
Source and more: Deadline