Mark is shocked to come face-to-face with his brother Damien in Thursday's Fair City on RTÉ One and the RTÉ Player.
The prison visit comes after Mairéad urges Damien not to turn his back on family.
Mark is in custody after being charged with the murder of his sister Laura - and a hostage standoff that saw Mairéad shot.
Fans can find out what happens next on Thursday on RTÉ One and the RTÉ Player at 8:30pm.
NB: There is no Fair City episode on Tuesday.
