I think there's a word or an adjective for it, but I don't remember what it was. I don't want to say plan B, because that's too informal, but I remember there was a good word for it. I just don't remember it anymore.
For example:
The army was flanked by the enemies unexpectedly so that the general had to rely on his ___ plan.
informal
,colloquial
, orslang
. (There’s nothing wrong with asking for synonyms, but it may not be as informal as you seem to think it is.) – J.R.♦ Apr 13 '19 at 20:02