Step into the breach
Definition 1 of 1
Pro Tip 1/3Figurative Use
The phrase has a figurative meaning; it's not about a physical step but a role or responsibility taken. 
With the chef absent, the sous chef had to step into the breach and manage the kitchen.
Pro Tip 2/3Temporary Action
This phrase often implies the action is a temporary solution, not a permanent role change. 
During the blackout, the security guard stepped into the breach and directed traffic.
Pro Tip 3/3Emergency Context
It is mostly used in an emergency or when someone is unexpectedly unavailable. 
When the goalkeeper got injured, a field player had to step into the breach.