We present our extensive observational campaign on the Swift-discovered GRB 141121A, almost 10 years after its launch. Our observations cover radio through X-rays and extend for more than 30 days after discovery. The prompt phase of GRB 141121A lasted 1410 s and, at the derived redshift of z = 1.469, the isotropic energy is E$_γ,iso$ = 8.0 × 10$^52$ erg. Due to the long prompt duration, GRB 141121A falls into the recently discovered class of ultra-long GRBs (UL- GRBs). Peculiar features of this burst are (1) a flat early-time optical light curve and (2) a radio-to-X-ray rebrightening around three days after the burst. The latter is followed by a steep optical-to-X-ray decay and a much shallower radio fading. We analyze GRB 141121A in the context of the standard forward- reverse shock (FS, RS) scenario and we disentangle the FS and RS contributions. Finally, we comment on the puzzling early-time (t łesssim 3 days) behavior of GRB 141121A, and suggest that its interpretation may require a two-component jet model. Overall, our analysis confirms that the class of UL-GRBs represents our best opportunity to firmly establish the prominent emission mechanisms in action during powerful gamma- ray burst explosions, and future missions (like SVOM, XTiDE, or ISS-Lobster) will provide many more of such objects.