Context. The mechanisms producing the diffuse radio emission in galaxy clusters, and in particular their connection with cluster mergers, are still debated. Aims: We seek to explore the internal dynamics of the cluster Abell 1758N, which has been shown to host a radio halo and two relics, and is known to be a merging bimodal cluster. Methods: Our analysis is mainly based on new redshift data for 137 galaxies acquired at the Telescopio Nazionale Galileo, only four of which have redshifts previously listed in the literature. We also used photometric data from the Sloan Digital Sky Survey and from the Canada-France-Hawaii Telescope archive. We combined galaxy velocities and positions to select 92 cluster galaxies and analyzed the internal cluster dynamics. Results: We estimate a cluster redshift of ⟨z⟩ = 0.2782 and quite a high line-of-sight (LOS) velocity dispersion σV ~ 1300 km s-1. Our 2D analysis confirms the presence of a bimodal structure along the NW-SE direction. We add several pieces of information to the previous merging scenario: the two subclusters (here A1758N(NW) and A1758N(SE)) cannot be separated in the velocity analyses and we deduce a small LOS velocity difference (ΔVrf,LOS ≲ 300 km s-1 in the cluster rest-frame). The velocity information successfully shows that A1758N is surrounded by two small groups and active galaxies infalling onto, or escaping from, the cluster. Removing the two groups, we estimate σV,NW ~ 1000 km s-1 and σV,SE ~ 800 km s-1 for A1758N(NW) and A1758N(SE), respectively. We find that Abell 1758N is a very massive cluster with a range of M = 2-3 × 1015 h70-1 M&sun;, depending on the adopted model. Conclusions: As expected for clusters that host powerful, extended, diffuse radio emissions, Abell 1758N is a major cluster merger just forming a massive system.