Ghost is episode 5 of Mirror Mirror Series 2 and episode 25 overall.
After Constance and her mother have their photo taken and Constance notices a mysterious figure in the background. The children have been hiding the mirror so that Lily can't go through. Fergus scans the photo negative into the computer to try and find out who the mysterious figure is. While Gervaise works down in the cellar strange things are occurring. The ghost then appears to Constance and writes "Beware". They discover that the ghost is that of Charles Jamieson who built Limerick House in 1842 when he came out from Ireland, and he died in the cellar. The ghost was warning them of a cave in that would happen during the upcoming earthquake in the cellar, luckily Constance goes through the mirror in time to save her mother from being crushed.