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We study the nearby lenticular galaxy NGC 3998. This galaxy is known to hosta low-power radio AGN with a kpc-size one-sided jet and a large, nearly polarHI disc. It is therefore a good system to study to understand the relationbetween the availability of cold-gas and the triggering of AGNs in galaxies.Our new WSRT data reveal two faint, S-shaped radio lobes extending out to$\sim$10 kpc from the galaxy centre. Remarkably, we find that the inner HI discwarps back towards the stellar mid-plane in a way that mirrors the warping ofthe radio lobes. We suggest that the polar HI disc was accreted through a minormerger, and that the torques causing it to warp in the inner regions are alsoresponsible for feeding the AGN. The "S" shape of the radio lobes would then bedue to the radio jets adapting to the changing angular momentum of the accretedgas. The extended radio jets are likely poorly collimated, which would explaintheir quick fading and, therefore, their rarity in galaxies similar to NGC3998. The fuelling of the central super-massive black hole is likely occurringvia "discrete events", suggested by the observed variability of the radio coreand the extremely high core dominance, which we attribute to the formation andejection of a new jet resulting from a recent fuelling event.

A rare example of low surface-brightness radio lobes in a gas-rich early-type galaxy: the story of NGC 3998