This invention relates to apparatus for setting a plug in, or retrieving a plug from, a horizontal xmas tree. The apparatus comprises a housing with a throughbore adapted to be releasably connected at a first end to a horizontal xmas tree, the housing including an at least one axially moveable member which also includes an at least one radially moveable element and a tool deployable within the housing throughbore by an elongate support and adapted to be releasably connected to the plug. The tool has an outer surface portion profiled to releasably engage with an engagement surface the at least one radially moveable element whereby, in operation, engagement of the at least one radially moveable element with the tool outer surface portion permits the plug, connected to the tool, to be set in the horizontal xmas tree or retrieved from the horizontal xmas tree by axial movement of the at least one axially moveable member.