NOVELTY - The system has a casing (10) comprising secondary fluid lines (15) running in a longitudinal direction parallel to an internal elongate hollow tubing (11). A plug-in device provides a different technical function in relation to production of primary fluid, and comprises a functional section e.g. wire line retrievable oil production (WROP) blind plug functional section, extending between adjacent sealing units, where the functional section comprises a drive for a pump, which is connectable to the drive such that the pump lies closer to a reservoir than the functional section in a position (P1).    USE - System i.e. WROP system, for producing primary hydraulic fluid i.e. oil (claimed), from an underground reservoir, utilized in oil and gas industry, geothermal industry and nuclear industry and onshore and offshore environments.    ADVANTAGE - The system allows the centrifugal displacement pump or a cavity displacement pump to be chosen, so that system costs can be reduced, and pump effects can be balanced such that mass production of the reservoir can be achieved and gas/water/emulsion can be reused for injection into hydra-lines. The system uses wire line techniques to pull the pump out of the tubing and change, repair and install the pump afterwards into the tubing to overcome malfunctions, so that loss of production time can be minimized, thus avoiding need for expensive work over towers, and hence providing noiseless functioning and environmental-friendly system.    DETAILED DESCRIPTION - The drive is a hydraulic drive or a turbo motor drive. The sealing unit is a packing unit. The plug-in device is a blind plug-in device, gas lift plug-in device or WROP -heat exchanger plug-in device. The pump is a plunger or piston pump, a positive displacement pump or a centrifugal displacement pump. INDEPENDENT CLAIMS are also included for the following:    (1) a plug-in device    (2) a method for producing primary fluid from an underground reservoir.    DESCRIPTION OF DRAWING(S) - The drawing shows a cross sectional view of a well bore in which a system produces primary fluid.    Positions (P1- P6)    Casing (10)    Internal elongate hollow tubing (11)    Secondary fluid line (15)    Well bore (100)