This paper presents a query-answering algorithm to compute minimal lines of defence around an individual argument. The algorithm returns all such defence sets together with an indication whether the defence is grounded or admissible. For every argument encountered in the search process the algorithm further indicates whether that argument is IN, OUT, or UNDEC (undecided) according to the grounded semantics. The presentation of the algorithm is followed by a correctness proof and a complexity analysis of other than worst cases. The algorithm is already functional in argument analysis and visualization tools.