An auto-injector, comprising: a first subunit or substructure (110) comprising: a chamber (112) for containing a fluid, so that the chamber (112) has an outlet opening (114) and an inner surface； and a transfer unit or structure (116) located - so that it can be moved - in the chamber (112) and having an outer surface basically in contact with the inner surface around its perimeter or contour, so that said transfer unit (116) is adapted to transfer fluid to the chamber (112) when the transfer unit (116) moves inside the chamber (112)； so that the transfer unit (116) is adapted to receive or accommodate a fluid container (120) and transfer the fluid from the container (120) to the chamber (112) when the transfer unit (116) travels with with respect to the camera (112)； such that the transfer unit (116) comprises a hollow fluid transfer needle (118) that is adapted to join the fluid container (120) and form a fluid path from the container (120) to the chamber (112) ) through the hollow needle (118)； characterized by the fact that: the transfer unit (116) also has a plug (128) that blocks the movement of fluid (s) to the outside of the transfer unit (116)； and so that the fluid transfer needle (118) is adapted to pierce the plug (128) in order to bring the fluid through the plug (128) to the chamber (112).Un autoinyector, que comprende: una primera subunidad o subestructura (110) que comprende: una cámara (112) para contener un fluido, de manera que la cámara (112) tiene una abertura de salida (114) y una superficie interior； y una unidad o estructura de transferencia (116) situada -de forma que se pueda mover- en la cámara (112) y que tiene una superficie exterior básicamente en contacto con la superficie interior alrededor de su perímetro o contorno, de manera que la citada unidad de transferencia (116) está adaptada para transferir fluido a la cámara (112) cuando la unidad de transferencia (116) se mueve dentro de la cámara (112)； de manera que la unidad de transfer
